Data transfer rate between communication products is ever-increasing. To ensure that transmission of a high frequency signal by a carrier is of high quality, the carrier, such as a print circuit board, or a cable such as flexible print circuit board/flat flexible cable (FPC/FFC), and Flex-rigid PCB, must have specified structure such as microstrip line, stripline and coplanar waveguide . . . etc so as for an electronic signal to be transmitted between conducting wires and to assure matching of characteristic impedance.
The communication products are usually equipped with flexible circuit boards, such as a flexible printed circuit board (flexible PCB) or a flex-rigid PCB, for functioning as a carrier on condition that a high speed signal's rise time, fall time, clock, jitter, and eye diagram are not compromised. A layout is formed on the carrier to function as a signal transmission structure, such as a Liquid Crystal Display (LCD) interface, a Charge-Coupled Device (CCD) interface, a Peripheral Component Interconnect (PCI), a Universal Serial Bus (USB) interface, a Serial Peripheral Interface (SPI) interface, an Inter-integrated Circuit (I2C) interface, an Audio Codec interface, a Local Area Network (LAN) interface, or an Advanced Technology Attachment (ATA) interface, for transmitting a high frequency signal to the carrier.
The foregoing signal transmission structure for transmitting a high speed signal to the carrier is likely to generate noise when coupled to another signals or transmit in a discontinuous structure, and thus the prior art is focused mostly on reduction of the harmonic of operation frequency. In general, the prior art solves the high frequency noise problem by increasing signal attenuation, avoid transmitting in a discontinuous structure, increase the spacing between signals and shielding the path of noise coupling and radiation. At present, suppression of signal attenuation is achieved in two ways: circuit compensation (or correction); and using materials which are unlikely to cause signal attenuation. The prior art is disclosed in Taiwan Patent No. I229497 and Taiwan Patent No. I246251, for example.
Considering the characteristics of circuits, the high-frequency part of a signal attenuates inevitably and readily; and the attenuation can be corrected by circuit design. For instance, a waveform correction circuit at the receiving end is configured to attenuate the low-frequency part of a signal received, so as to render signal attenuation consistent and thereby prevent waveform distortion. The otherwise reduced amplitude of the signal is restored by a signal amplifier at the back end. The input impedance of the correction circuit has to match characteristic impedance so as to prevent the waveform and signal from deteriorating.
For example, the prior art disclosed a low-pass filter disposed between a signal transmitting end and a signal receiving end for filtering out high frequency noise.
Nonetheless, a low-pass filter cannot operate without greatly increasing the area of a chip working and affect the requirement of signal quality in conjunction with the low-pass filter. Furthermore, under a variable process, a delay time of a delay unit is intractable and inflexible, for example. In addition, capacitors and resistors occupy a considerable part of the area of an integrated circuit and are indispensable to the low-pass filter. Hence, the presence of the low-pass filter inevitably requires a great increase in the area of the chip. Also, the delay time of the delay unit is not free from errors due to process variation, and nothing can be done to deal with the errors in the design phase. The disadvantageous outcome spares neither performance nor system stability. Last but not least, given the aforesaid design, the fixed, inflexible delay time is unlikely to work well with input signals at different frequencies, thereby compromising system stability.
In the situation where a substrate is made of a material unlikely to cause signal attenuation, loss of a high frequency signal can be suppressed by a low dielectric constant and small dielectric loss. However, such a substrate is rare and expensive. Hence, using the substrate for reducing the loss of a high frequency signal, punctual delivery and price should be evaluated carefully.
As explained above, drawbacks of the prior art abound. In this regard, an urgent issue involves developing a technology to improve carrier-based signal transmission and thereby overcome the drawbacks of the prior art.